In the typical RIM operation, a mold cavity formed by at least two mating mold parts is filled with reactive chemicals that are mixed and injected at high pressure into the mold cavity, wherein an exothermic polymerization reaction substantially increases the pressure within the cavity. During the reaction process, it is important to clamp the mold parts firmly together to prevent the material being molded from escaping at the junctures between the mold parts. The RIM of large products requires tremendous compression forces over a comparatively large area, such that conventional presses used for high tonnage RIM operations tend to warp or deform during the molding process. Although the press beds between which comparatively large mold parts are pressed comprise heavy rigid steel structures, it has not been economically feasible to provide such presses that will not deform. In consequence, the force applied to clamp the mold parts together is distributed unevenly over the area of the mold, enabling the extrusion of pressurized reacting chemicals through tiny spacings at the mold junctures. Such spacings on the order of a thousandth of an inch are significant, may result in improperly formed molded products, and in any event necessitate an additional operation to remove flash from the molded product.
Some RIM molds are characterized by a female mold part having a deep cavity that cooperates with a male mold part having projections that extend deeply, i.e., as much as 30 inches or more, into the cavity when the mold parts are brought together to interfit at a mold closed position. The mold cavity is usually defined by highly polished and accurately machined surfaces. Accordingly, not only must the junctures between the mold parts at their parting surfaces be tightly sealed during the high pressure molding process, but each part of such molds must be orientated precisely with respect to a common reference plane, which is usually horizontal when the mold parts move vertically between their open and closed mold positions. If the supporting structure for either mold part becomes tilted slightly from the reference plane, the projections of the male mold part that extend deeply into the female cavity might contact and damage the mold and in any event, will alter and possibly cause a defective molded part. It is accordingly essential to successful operation with such RIM molds that the supports for the mold remain precisely parallel to the reference plane as the mold parts approach the mold closed position.
Although the prior art relating to molding presses is extensive, very little of that art known to applicant is concerned with the problem of preventing or compensating for deformation of the press components. The patent to Hammon, U.S. Pat. No. 4,304,540, is typical of a conventional type of press that ignores the deformation problem and is thus limited to the molding of small products involving comparatively low pressure applications. The Hammon press is concerned with the SCM industry (sheet moulding compounds) wherein extrusion of fluid high pressure chemicals from the mold seams is not an important problem. In the SCM operation, material such as a sheet or blank to be formed is placed on a lower open die part and thereafter shaped by a high tonnage forming operation that requires several inches of relative travel of upper and lower die parts toward each other.
Hammon provides stress or clamping rods 18 mounted at the corners of fixed and movable press beds or mold supports 12 and 24 respectively. A locking assembly 22 carried by the movable bed 24 clamps the serrations 21 of each rod 18 to lock the movable bed 24 adjacent to a forming or "reference" position prior to application of the high tonnage forming pressure. Thereafter pressure is applied to the upper sides of pistons 57, FIG. 1, which are secured to the rod 18 to pull the latter and bed 24 downwardly in a forming operation. After the forming operation, pressure is applied to the lower ends of pistons 57 to effect a stripping action by pushing the rods 18 upwardly.
Such a press is suitable for use only with comparatively small molds because under extremely high tonnage force, in addition to deformation of other press components, the locking rods 18 are stretched, usually non-uniformly. Although the corner portions of the beds 12 and 24 are tightly clamped together, their central portions, when subjected to the high tonnage molding conditions of a RIM process, are insufficiently clamped to the extent that the high pressure reactive chemicals being molded extrude from the mold as flashing.
The patents to Quere et al, U.S. Pat. No. 2,916,768 and Larson et al, 4,318,682, recognize the problem of deformation and the possibility of improperly aligned press components. Quere '768, for example, provides for independent adjustment of the corner mounted stress rods 5 and for the use of different pressures in the actuating cylinders 3 to compensate for such deformation. Such a mold requires sophisticated controls and at best can only minimize deformation when the press is used with comparatively small molds. Even if the clamping forces at the corners of the mold are equalized, the mold will still be subject to the disadvantages of the corner mounted clamping devices used by Hammon.
The Larson et al patent makes an attempt to compensate for deformation of press components by distributing the high pressure cylinders 38 over the area of the mold. The patent also discloses the use of replaceable spacer shims 66 engageable with stop 64 to predetermine the spacing between the fixed support 18 and movable platen 16 at its lowermost position, whereby the locking plates 80, 80' properly engage one of the slots in 60. However the structures disclosed are inadequate for RIM applications and are unrelated to the concept of the invention disclosed herein.
Typically, of SMC presses, the lowermost position of Larson's platen 16 is not comparable to the closed position of a RIM type press. Instead, it is equivalent to Hammon's "reference" position wherein the movable platen is locked in a position that determines the start of the high tonnage forming operation. The high tonnage platen must then move several inches at high pressure beyond the "reference" position to form the sheet or plate between the dies, column 3, lines 30-34 of Larson et al.
Larson's upper platen 16 in effect becomes a fixed platen after it is moved to and locked at its "reference" or lowermost position which determines the beginning of the sheet forming operation. High tonnage force and appreciable hydraulic power must then be applied by the cylinders 38 to move the lower platen 14 upwardly in a forming operation against the fixed platen 16. The mold parts of a RIM press on the other hand are moved by comparatively low power means to a closed position in contact with each other. Thereafter, high tonnage force is only required to hold the mold closed sufficiently tightly during the molding operation to prevent extrusion of the pressurized chemicals from within the mold. Inasmuch as the mold in a RIM operation is already closed when the high tonnage pressure is applied, only a minuscule amount of high pressure hydraulic fluid is required to hold the mold closed.
Another type of press known to the art and concerned with the provision of a uniform distribution of molding force over the area of a mold is variously known as a bladder or pillow type press. Such presses provide a high pressure chamber having a movable and usually deformable wall coextensive with a movable mold plate and deformable against the latter to clamp it toward a fixed mold plate during a molding operation. Typically, high speed means are also provided for moving the movable plate and high pressure chamber in unison to and from a mold closed position whereat the movable mold plate is adjacent to the fixed mold plate and in position to carry out the molding operation upon the injection of pressurized fluid into the high pressure chamber. Such presses are only suitable for molding products having comparatively small surface areas requiring a comparatively small total molding force, wherein deformation of the press components is not a problem and high pressure stripping is not required. The deformable wall of the high pressure chamber can only exert a unidirectional molding force and is thus not suitable for high pressure stripping.
A typical pillow or bladder type press is disclosed in the Descrovi et al U.S. Pat. No. 4,247,278, which recognizes the problem of deformation of the mold plates and provides a fluid pressurized cylinder 77 having an axial end wall 76, FIGS. 1, 2, or 216, FIGS. 3, 4, sufficiently thin and flexible to conform to deformation of an adjacent mold plate when the cylinder 77 is pressurized during a molding operation. Descrovi et al, like other pillow or bladder type patents, is not suitable for high tonnage operation. At the outset, it does not enable high pressure stripping by the same pressure exerting system that provides the mold closing pressure. Also, the area of the deformable walls 76 and 216 must be strictly limited. Otherwise these walls will be ruptured if subjected to the high pressure RIM of a large product. The deformable wall must be sufficiently thin to conform to deflection of the adjacent mold carrier and must be sufficiently thick to prevent its destruction within cylinder 77. Accordingly bladder or pillow type presses such as Descrovi et al must be operated within a comparatively limited range of clamping pressure.